Investigation of molten salt electrolytes for low-temperature liquid metal batteries

Abstract

This thesis proposes to advance our ability to solve the challenge of grid-scale storage by better positioning the liquid metal battery (LMB) to deliver energy at low levelized costs. It will do this by rigorously developing an understanding of the cost structure for LMBs via a process-based cost model, identifying key cost levers to serve as filters for system down-selection, and executing a targeted experimental program with the goal of both advancing the field as well as improving the LMB's final cost metric. Specifically, cost modelling results show that temperature is a key variable in LMB system cost as it has a multiplicative impact upon the final $/kWh cost metric of the device. Lower temperatures can reduce the total cost via simultaneous simplifications in device sealing, packaging, and wiring. In spite of this promise, the principal challenge in reducing LMB operating temperatures (>400°C) lies in identifying high conductivity, low-temperature electrolytes that are thermally, chemically, and electrochemically stable with pure molten metals. For this reason, a research program investigating a promising low-temperature binary molten salt system, NaOH-NaI, is undertaken. Thermodynamic studies confirm a low eutectic melting temperature (219°C) and, together with the identification of two new binary compounds via x-ray diffraction, it is now possible to construct a complete phase diagram. These phase equilibrium data have then been used to optimize Gibbs free energy functions for the intermediate compounds and a two-sublattice sub-regular solution framework to create a thermodynamically self-consistent model of the full binary phase space. Further, a detailed electrochemical study has identified the electrochemical window (>2.4 V) and related redox reactions and found greatly improved stability of the pure sodium electrode against the electrolyte. Results from electrochemical studies have been compared to predictions from the solution model and strong agreement supports the physicality of the model. Finally, a Na[/]NaOH-NaI[/]Pb-Bi proof-of-concept cell has achieved over 100 cycles and displayed leakage currents below 0.40 mA/cm℗ø. These results highlight an exciting new class of low-melting molten salt electrolytes and point to a future Na-based low-temperature system that could achieve costs that are 10-15% less than those of existing lithium-based LMBs.